A known inverting system for inverting direct current (DC) to alternating current (AC) relies on the bang-bang control of an inverter switch that receives DC at an input and provides AC at an output. A bang-bang control in this application is robust and provides good reference tracking performance. However, the switching frequency of a bang-bang control arrangement is high. This high switching frequency stresses the inverter switch of the inverter and also cause high power loss in the inverter switch. Moreover, in a bang-bang control arrangement, the switching frequency is not predictable.
On the other hand, an inverting system 10 as shown in FIG. 1 reduces the high switching frequency stress and high power loss of an inverter switch by reducing switching frequency. The inverting system 10 includes a DC source 12, an inverter switch 14, a control 16, and a filter 18. The control 16 controls the inverter switch 14 in order to invert the DC from the DC source 12 to AC for supply to a load 19. The filter 18 shapes and filters the AC provided at the output of the inverter switch 14 in order to produce sinusoidal AC at the output of the inverting system 10.
In FIG. 2, the control 16 is shown in more detail as a conventional pulse width modulator that receives a reference 20, and the individual elements of the filter 18 and the load 19 are represented as transfer functions. The reference 20 is usually in the form of a sinusoidal signal. A first summer 22 of the control 16 subtracts a feedback from the reference 20 in order to produce an error that is processed by a compensator 24. This feedback is derived from an output signal V.sub.C on an inverting system output 26 that is processed by a gain block 28. The gain applied by the gain block 28 is also referred to as a scale factor. The compensator 24 is arranged to stabilize the closed loop system of FIG. 2. A second summer 30 subtracts a triangular wave form 32 from the output of the compensator 24 in order to produce a pulse width modulation signal that is applied to a control input of the inverter switch 14. The inverter switch 14 responds to the pulse width modulation signal in order to convert the DC from the DC source 12 to AC. The output AC of the inverter switch 14 is then filtered by the filter 18 in order to generate sinusoidal AC on the inverting system output 26. This sinusoidal AC on the inverting system output 26 is supplied to the load 19.
If the output signal V.sub.C on the inverting system output 26 matches the reference 20 both in magnitude and phase, the error provided by the first summer 22 is zero. Accordingly, the output pulses at the output of the second summer 30 have a 50% duty cycle. However, as the output signal V.sub.C on the inverting system output 26 varies from the reference 20 in magnitude and/or phase, an error is generated at the output of the first summer 22 which causes the duty cycle of the pulses at the output of the second summer 30 to vary from a 50% duty cycle.
Unfortunately, the filter 18 of the inverting system 10 and the load 19 introduce both an attenuation and a phase shift in the output voltage on the inverting system output 26 with respect to the reference 20 so that the output signal V.sub.C does not quite match the reference 20.
The present invention is directed to a control in which the attenuation and phase shift imposed by the inverting system 10 is predicted so that the predicted attenuation and phase shift can be used to improve the reference tracking capability of the inverting system. Accordingly, the present invention combines the advantages of good reference tracking performance of a bang-bang controller and the low frequency switching characteristics of triangular pulse width modulation.